It is well known to insert a bullet-like tool or mandrel into a heat-transfer tube and then displace the tool axially with respect to the tube so as to effect radial expansion of the tube into gripping engagement with surrounding heat-transfer fins. However, in the forming of such heat-transfer members, namely, heat-transfer tubes having radially extending fins thereon, the bullet-like tool is conventionally fixedly secured to a pushing or pulling element and effects solely a radial expansion of the tube because the tool has a diameter slightly greater than the normal interior diameter of the tube. While this forming apparatus and process are satisfactory, nevertheless there is a continuing desire to improve the performance or heat-transfer characteristics of heat-transfer elements formed in this way. The apparatus and process of this invention are effective to significantly improve such heat-transfer characteristics.
Accordingly, it is an object of this invention to provide an improved apparatus and process for forming heat-transfer elements wherein platelike fins are secured to the outside wall of a heat-transfer tube. In the improved process and apparatus of this invention, there is provided a bullet-shaped tool or mandrel which is axially slidably inserted into the heat-transfer tube to effect radial expansion thereof into gripping engagement with the surrounding platelike fins. In a preferred embodiment of the invention, the tool is freely rotatably supported on the end of a suitable pushing or pulling rod, and the peripheral surface of the tool has a large number of small grooves extending axially therealong, which grooves are also skewed relative to the axial direction, that is, provided with a helixed or spiral orientation. These grooves hence define a large number of small teeth on the periphery of the tool, and the outer diameter of these teeth is slightly greater than the internal diameter of the tube. By axially pushing or pulling the tool into the tube, the teeth bite into the internal wall of the tube and, due to the helical orientation of these teeth, the axial displacement of the tool into the tube causes the tool to simultaneously rotate. This causes not only radial expansion of the tube for gripping the fins, but simultaneously results in the formation of spiral-like grooves on the internal wall of the tube. These grooves increase the surface area of the internal surface of the tube and increase the turbulence of the heat-transfer liquid, and thus increase the heat-transfer characteristic of the finned tube.
While a spirally grooved tool or mandrel movable internally of a workpiece is already known, as evidenced by U.S. Pat. No. 4,300,275 (McLaughlin), nevertheless this known tool is utilized for forming a wholly different type of structure, and the tool possesses a mounting structure which is less than desirable when one wishes to repetitively form large numbers of heat-transfer tubes, as in the present invention.
Hence, the present invention provides an improved bullet-like tool having grooves on the periphery thereof, with the tool having an improved mounting arrangement which facilitates the rotatable mounting of the tool on the free end of the pushing or pulling rod so as to enable it to possess substantial durability and at the same time withstand the substantial forces and the working conditions to which it is exposed.
The present invention also relates to an improved process for forming finned heat-exchanger tubes using the improved tool described above, which process permits the turbulence-causing spiral grooves to be formed on the internal wall thereof simultaneously with effecting the radial expansion of the tube into engagement with the surrounding fins, whereby the overall finned heat-exchanger tube can hence be formed in a single operation.
Other objects and purposes of the invention will be apparent to persons familiar with apparatus and processes of this general type, after reading the following specification and inspecting the accompanying drawings.